Novel magnetic bimetallic AuCu catalyst for reduction of nitroarenes and degradation of organic dyes

Herein, core–shell magnetic nanoparticles are modified with imidazolium-tagged phosphine and propylene glycol moieties and used for the stabilization of bimetallic AuCu nanoparticles. The structure and morphology of the prepared material are identified with SEM, TEM, XRD, XPS, atomic absorption spectroscopy, Fourier translation infrared spectroscopy, and a vibrating sample magnetometer. This hydrophilic magnetic bimetallic catalyst is applied in the reduction of toxic nitroarenes and reductive degradation of hazardous organic dyes such as methyl orange (MO), methyl red (MR), and rhodamine B (RhB), as well as in the degradation of tetracycline (TC). This magnetic AuCu catalyst indicated superior activity in all three mentioned reactions in comparison with its single metal Au and Cu analogs. This catalyst is recycled for 17 consecutive runs in the reduction of 4-nitrophenol to 4-aminophenol without a significant decrease in catalytic activity and recycled catalyst is characterized.

www.nature.com/scientificreports/for removing dyes from industrial effluents 25 .Tetracycline (TC) is another damaging contaminant, which is broadly utilized as antibiotic in the bacterial infections treatment, agriculture, and fish farming.Released TC can be stay in the environment for a long time due to its high chemical stability, leading to increase the resistance of microorganisms, disruption of photosynthesis in aquatic organisms, and acute toxicity to humans [26][27][28] .Advanced oxidation and adsorption are the two main and accepted methods for eliminating tetracycline from wastewater 29 .
In addition, nitroaromatic compounds, especially nitrophenols, are among the large industrial wastes and pose a high risk to human and animal health due to their mutagenic and carcinogenic potential 30 .Numerous methods have been used to decompose or convert nitroarenes, with the catalytic reduction of them being the most suitable approach.Interestingly, the produced amines are useful building blocks for the synthesis of antioxidants, drugs, pharmaceuticals, agrochemicals, dyes, pesticides, and polymers 31 .Among different efficient, economical, and environmentally friendly processes for removing pollutants from the environment, much attention has been paid to the employment of heterogeneous catalysts with superior activity and good recyclability using conventional methods such as centrifugation and filtration, which are time-consuming, wasteful, and industrially inefficient 32 .
Herein, we report a novel phosphine, ionic liquid, and diol-modified magnetic nanoparticles supported AuCu species, Fe 3 O 4 @Phos-IL-AuCu, as an extremely water-dispersible and magnetically recyclable catalyst with high efficiency in the reduction of nitroarenes, organic dyes, and degradation of tetracycline.

Result and discussion
The complete procedure for the preparation of the titled bimetallic catalyst is detailed in Scheme 1.The freshly prepared Fe 3 O 4 nanoparticles, obtained from the reaction of FeCl 2 ⋅4H 2 O and FeCl 3 ⋅6H 2 O, were treated with Si(OEt) 4 to afford Fe 3 O 4 @SiO 2 core-shell.Next, (3-glycidyloxypropyl)trimethoxysilane was used to introduce the epoxy group (I).Then, the synthesized 1-(2-propynyl)-1H-imidazole (II) and chlorodiphenylphosphine were allowed to react with epoxy-functionalized magnetic nanoparticles(I) to achieve phosphorous and IL-modified MNPs.The alkyne group was then subjected to CuAAC reaction to introduce diol group.Finally, modified MNPs were treated with Au and Cu salts followed with reaction with NaBH 4 to give AuCu supported on modified MNPs.This material referred to as Fe 3 O 4 @Phos-IL-AuCu NPs throughout of this article.
The contents of Au and Cu in the Fe 3 O 4 @Phos-IL-AuCu were quantified by ICP to be 0.03 and 0.07 mmol/g, respectively.Every stage of the preparation of Fe 3 O 4 @Phos-IL-AuCu is analyzed by FT-IR spectroscopy, as shown in (Fig. 1).Peaks located at 461 cm −1 , 570 cm −1 , and 638 cm −1 corresponded to the Fe-O vibrations.Signals positioned at 800 cm −1 , 954 cm −1 , and 1119 cm −1 are derived from the Si-O, Si-OH, and Si-O-Si vibrations, respectively 33,34 .Broad bands centered at 2867 cm −1 and 2927 cm −1 were caused by C-H stretching vibrations 35 .The spectra of Fe 3 O 4 @SiO 2 -OPPh 2 -propargylimidazole and Fe 3 O 4 @Phos-IL compounds reveal that the peak at 1650 cm −1 is related to the C=N and C=C bonds of imidazole [36][37][38] .However, this peak overlaps with the bending vibration of absorbed water molecules.Additionally, the present peak at 1071 cm −1 in the spectrum of these compounds is due to the vibrations of the P-O bond, which is overlapping with the peak corresponding to the stretching vibration of the Si-O-Si bond 39 .In addition, the peak at 2120 cm −1 in the spectrum of Fe 3 O 4 @ SiO 2 -OPPh 2 -propargylimidazole is ascribed to the existence of the terminal alkyne group 40 .The disappearance of this peak in the final stage of the compound (Fe 3 O 4 @Phos-IL) signifies the successful completion of the CuAAC 1,3-dipolar reaction occurred.In the Fe 3 O 4 @Phos-IL spectrum, the peak at 3138 cm −1 is caused by the stretching vibration of the C-H bond of the triazole moiety, while the peak at 1636 cm −1 is the results from overlapping bending vibrations of water, as well as C=C and C=N vibrations of imidazolium 41,42 .In all cases, the wide band centered at 3400 cm −1 is associated to O-H stretching vibration 43 (Fig. 1).
The results of transmission electron microscopy (TEM) analysis of Fe 3 O 4 @Phos-IL-AuCu NPs samples revealed a distinct core-shell structure, with evidence of the presence of AuCu nanoparticles on the surface of the substrate.The determined average size of the AuCu NPs is 1.1 nm (see Fig. S1, Fig. 3).www.nature.com/scientificreports/ The scanning electron microscopy (SEM) images revealed the formation of Fe 3 O 4 @Phos-IL-AuCu NPs nanoparticles with a uniform distribution, good dispersion, and near spherical morphology (Fig. 4).
X-ray photoelectron spectroscopy (XPS) of Fe 3 O 4 @Phos-IL-AuCu in Fe, Si, C, N, Cl, P, Au, and Cu areas was analyzed (Fig. 5).The XPS plot of the Fe 2p region showed peaks at 709, 710, 712.5, and 716 eV, which correspond to Fe 2p3/2.Also, in this spectrum, the peaks in the regions of 723.9, 725.5 and 728.5 eV corresponded to Fe 2p1/2 (Fig. 5a) [44][45][46] .In the Si 2p spectroscopy, the peaks at 101.6, 102.8, and 103.5 eV corresponded to Si-C, Si-O-C, and O-Si-O, respectively (Fig. 5b) 47,48 .XPS analysis of C 1s plot showed peaks located at 284.8, 286, and 288.5 eV which were ascribed to C-C, C-OH, and N-C=N, individually (Fig. 5c) 49,50 .In the N 1s spectroscopy, the peak at 399.5 eV was associated to the C-N bond, the 400 eV peak was attributed to the C=N bond, and the peak at 401.6 eV in this region was associated to the NR 3 + species in imidazolium (Fig. 5d) [51][52][53] .In the Cl 2p spectral analysis, the peaks at 197.5, 199.4,and 201.7 eV were attributed to the Cl 2p3/2, while the peak at 200.4 eV was associated with the Cl 2p5/2 (Fig. 5e) [54][55][56][57] .XPS of phosphorus revealed two peaks in the 2p region, located at 131.2 and 133.4 eV, corresponding to the bonding states of P-C and P-O, respectively (Fig. 5f) 58,59 .Additionally, the XPS plot of the gold 4f area revealed two peaks located at 83.5 and 87.5 eV, both of them conforming to the Au(0) (Fig. 5g) 60 .Finally, the XPS analysis was performed on the Cu 2p area, revealing two signals at 931.5 and 932 eV, corresponding to Cu 2 O.The peak observed at 933.5 eV was attributed to Cu 2p3/2, while the peaks at 953.4 and 955 eV were assigned to Cu 2p1/2 in CuO.Additionally, smaller peaks in the regions of 935, 940, and 943 eV were attributed to metallic copper(II), copper(II) oxide and copper(II) hydroxide (Fig. 5h) [61][62][63] .
The superparamagnetic behavior of Fe 3 O 4 @Phos-IL-AuCu NPs was studied by vibrating-sample magnetometer (VSM) in which the results indicated zero coercivity together with remanence on the magnetization-loop, proving the superparamagnetic nature of the material and the possibility of easy separation by an external magnetic field (Fig. 7).
After the characterization of Fe 3 O 4 @Phos-IL-AuCu NPs, its catalytic activity was initially assessed in the reduction of nitro aromatic compounds.The reduction of the starting compound, 4-chloronitrobenzene, was  www.nature.com/scientificreports/selected as the reference reaction and various factors like type of reductant, solvent, catalyst loading (mol%), and reaction time were analyzed (Table 1).Initial results indicated that a 50% GC yield was obtained by using the catalyst with 0.06 mol% Au, 0.16 mol% Cu and using NaBH 4 (2 mmol) in H 2 O during 2 h at room temperature (Table 1, entry 1).Results showed that with the enhancement of the catalyst amount to Au (0.1 mol%) and Cu (0.24 mol%), a 65% yield of the desired amine was obtained (Table 1, entry 2), while with higher amounts of Au (0.15 mol%) and Cu (0.35 mol%), a quantitative yield was achieved (Table 1, entry 3).Additionally, the yield of the reaction involving 4-chloroaniline decreased with lower amounts of NaBH 4 (1 mmol) (Table 1, entry 4)   were selected as the optimal reaction conditions.This reference reaction, in the absence of the catalyst, did not make any progress (Table 1, entry 13).To highlight the advantage of using bimetallic Fe 3 O 4 @Phos-IL-AuCu, we examined the activity of single-metal catalysts, Fe 3 O 4 @Phos-IL-Au and Fe 3 O 4 @Phos-IL-Cu, in this reaction and using the optimized conditions.The experimental results indicated that both single-metal catalysts offered poorer yields than the reaction performed with the bimetallic catalyst (Table 1, entries 14, and 15).The higher catalytic activity of Fe 3 O 4 @Phos-IL-AuCu, as these results indicate, is due to the synergistic effect between the Cu and Au species.www.nature.com/scientificreports/After determination of optimum reaction conditions, the reduction of different nitroarene compounds was assessed (Table 2).The results showed that high to excellent yields were obtained in the reduction of nitro aromatic compounds with both groups of electron-donating groups such as -Me, -CH 2 OH, NH 2 , and -OH and electron-withdrawing groups such as -F, -Cl, -Br, -CN, and -COMe.In general, the results show that the reduction of electron-rich nitroarenes was slower than the substrates containing electron-deficient arrangements (Table 2, entries 2-15).Also, in the example run with 4-nitrobenzaldehyde, the aldehyde group was reduced along with the nitro group, (Table 2, entry 13).However, during the reduction of acetylnitroarenes only the nitro group was reduced (Table 2, entries 14, 15).In addition, nitro aromatic compounds with two nitro groups were completely reduced, using a larger amount of NaBH 4 over longer times (Table 2, entries 16-19).Also, compounds with both electron-donating and electron-withdrawing substitutions had good yields (Table 2, entry 20).Reduction of 4-nitrobenzylchloride was performed very good and the corresponded amine obtained in excellent yield (Table 2, entry 21).Finally, the reduction of heterocyclic nitro compounds was carried out efficiently and the expected amines were isolated in 83-89% yields (Table 2, entries 22-24).Besides, the carbonyl group of the amide remained intact (Table 2, entries 22 and 23).Totally results of Table 2 showed achieving high TOF and TON of present bimetallic AuCu catalyst in reduction of nitroarenes.
The catalytic performance of Fe 3 O 4 @Phos-IL-AuCu in the reductive degradation of organic dyes, including methyl red (MR), methyl orange (MO), and rhodamine B (RhB) was evaluated.The progresses of reactions were recorded by using UV-Vis spectrophotometer.The absorption of all MR (λmax = 435 nm), MO (λmax = 467 nm), and RhB (λmax = 556 nm) dyes decreased in time when using Fe 3 O 4 @Phos-IL-AuCu as catalyst together with NaBH 4 as reducing agent.Generally, MO (λmax = 467 nm), MR (λmax = 435), and RhB (λmax = 556 nm) dyes completely declined in 3 min, 2 min, and 1 min, respectively, and during the desired times all dyes became colorless (Fig. 8a-c) 70,71 .Diagrams of ln (At/A0) vs. reaction time for the dyes reduction were drawn and the rate constants (k) of MO, MR, and RhB were 1.5, 2.1, and 2.6 min −1 , respectively, which indicates good performance of the titled synthesized catalyst (Fig. 8d).TOF and TON for the reduction of dyes are calculated and shown in Table S2.
In order to compare the activity of the prepared bimetallic catalyst with the activity of the single metal catalysts such as Fe 3 O 4 @Phos-IL-Au and Fe 3 O 4 @Phos-IL-Cu, the reduction of dyes MO, MR, and RhB was investigated (Fig. S2).The results showed that 31.5%,27%, and 11.5% yields were obtained in the reduction of MO, MR, and RhB dyes when using the Fe 3 O 4 @Phos-IL-Au catalyst.Also, 5%, 3%, and 9% yields were obtained in the reduction of MO, MR, and RhB dyes with the Fe 3 O 4 @Phos-IL-Cu catalyst.These results confirmed the higher catalytic activity of Fe 3 O 4 @Phos-IL-AuCu due to the synergetic effect between Cu and Au species.
The proposed mechanisms for the reduction of nitroarenes and organic dyes using the Fe 3 O 4 @Phos-IL-AuCu catalyst are based on the well-established principles of metal-catalyzed hydrogenation, as depicted in Figs.S6 and S7.The process begins with reaction of NaBH 4 with H 2 O and generation of hydrogen and NaBO 2 .Produced hydrogen, along with the nitroarenes or organic dyes, are adsorbed onto the surface of the Fe 3 O 4 @Phos-IL-AuCu catalyst and performed the reduction of functional groups 72,73 .
Finally, the catalytic activity of the as-prepared Fe 3 O 4 @Phos-IL-AuCu catalyst has been assessed in the degradation of tetracycline (TC) by using ammonium peroxodisulfate at diverse pH levels (3, 5, 7, 9, and 11) (Fig. 9a-e).The reaction process was recorded using the UV-Vis spectrophotometer.Results showed that the absorbance of tetracycline gradually decreased with increasing reaction time, and, similarly to the experience in previous reports, the best result was obtained in alkaline media.
Degradation of tetracycline with Fe 3 O 4 @Phos-IL-Au (Fig. S3) and Fe 3 O 4 @Phos-IL-Cu (Fig. S4) was also investigated.Here, the higher efficiency of bimetallic catalyst Fe 3 O 4 @Phos-IL-AuCu is demonstrated once more versus the reactions run in the presence of monometallic catalytic species (Table S1).
Since the reduction of 4-nitrophenol proceeded very efficiently (Table 2, entry 8), we studied the synthesis of acetaminophen via a one-pot reduction/amidation of 4-nitrophenol.Results show that the reaction proceeded very effectively and the desired product was obtained in 84% isolated yield (Scheme 2).
Efficiently recovering and reusing magnetic heterogeneous catalysts is crucial for both environmental and economic perspectives.In this regard, the recovery and the recycling facilities of Fe 3 O 4 @Phos-IL-AuCu were studied in the reduction of 4-nitrophenol under optimized reaction conditions (Fig. 10).To achieve this objective, the catalyst was recovered after 30 min using an external magnet, washed with ethyl acetate, and after drying, it was utilized in another new batch of the reaction.According to these findings, the catalyst demonstrated the ability to be reused for 16 consecutive runs with only a slight decrease in activity.However, during the 17th run, the yield dropped to 81%.
To determine whether the Fe 3 O 4 @Phos-IL-AuCu is heterogeneous or homogeneous in nature, the filtration test was used for the reduction of 4-chloronitrobenzene under optimal conditions.During the course of the experiment, the reaction proceeded to a level of 39% completion after an initial reaction time of 1 h.At this stage, the reaction mixture was subjected to filtration, following which the filtrate was allowed to continue the reaction for an additional hour.Subsequent analysis using gas chromatography (GC) revealed that the progress of the reaction was slowed down and the product was formed at a yield of only 49% (Fig. 11).This result indicated negligible leaching of active catalysts into the reaction mixture and the possible heterogeneous nature of the catalyst.
In order to investigate the stability of the catalyst, the structure of the reused catalyst was analyzed by VSM (Fig. 7), XPS (Fig. 12), XRD (Fig. 13), and TEM (Fig. 14).VSM analysis of the reused catalyst showed the preservation of the magnetic property of the catalyst during recycling.The information revealed by the XPS spectrum in the Fe, Si, C, N, Cl, P, Au, and Cu regions indicated that similar patterns of elements to those of the fresh catalyst existed, confirming the high stability of the catalyst during the reaction.www.nature.com/scientificreports/Furthermore, the XRD spectra obtained from the reused catalyst showed peaks corresponding to Fe 3 O 4 , Si, Au, and Cu, which are close to the analogous ones detected in the newly prepared catalyst, providing evidence of the catalyst's durability (Fig. 13).www.nature.com/scientificreports/TEM images of reused catalysts also showed a very similar pattern to that of fresh catalysts with small increase in average size of nanoparticles, confirming the stability of the catalyst (Fig. 14, Fig. S5).

Synthesis of Fe 3 O 4 NPs
Firstly, FeCl 3 ⋅6H 2 O (16 mmol, 4.3 g), FeCl 2 ⋅4H 2 O (8 mmol, 1.6 g), and distilled water (70 mL) were mixed and stirred under an argon atmosphere at room temperature for 10 min.After that, aqueous ammonia (25%, 10 mL) was gradually poured, and the suspension stirred at 80 °C for 4 h.Finally, an external magnet allowed for the separation of the magnetic particles which were washed with distilled H 2 O (3 × 20 mL) and EtOH (3 × 20 mL), and dried in the oven at 60 °C.

Synthesis of Fe 3 O 4 @SiO 2 @epoxide (I)
Freshly obtained Fe 3 O 4 @SiO 2 (1 g) was suspended in dry toluene (30 mL) and then sonicated for 30 min.Next, (3-glycidyloxypropyl)trimethoxysilane (5 mmol, 1.1 mL) was added to the above mixture under argon atmosphere.The suspension was stirred at 105 °C for 1 day.The Fe 3 O 4 @SiO 2 @epoxide product was separated magnetically, washed repeatedly with distilled water and dichloromethane, and lastly dried at 60 °C for 4 h.Sodium hydride (60%, 9 mmol, 360 mg) was added to a 25 mL flask under argon protection.Then, dry THF (6 mL) was added and flask was placed in an ice bath.Next, crystallized imidazole (9 mmol, 612 mg) was dissolved in 2 mL of dry THF and added to the mixture.After 5 min, propargyl bromide (7.5 mmol, 0.56 mL) diluted in 1 mL THF was added and mixture was stirred at room temperature for 24 h.Afterward, solvent was removed using a rotary evaporator and water (10 mL) and CH 2 Cl 2 (10 mL) were added to the residual mixture.The organic phase containing the product was separated and its solvent was evaporated.This washing process with water was repeated three additional times.Finally, an amber-colored product (II) was obtained.

Synthesis of Fe 3 O 4 @Phos-IL
Sodium azide (8 mmol, 0.52 g) and 3-Chloro-1,2-propanediol (5 mmol, 0.4 mL) were dissolved in N,N-dimethylformamide (10 mL), and then the reaction mixture was stirred at 100 °C for 24 h.In the next step, the mixture was cooled to room temperature and the phosphine functionalized magnetic NPs (1 g) were added to the mixture.Then, a solution containing copper(II) sulfate (0.09 mmol, 14.3 mg) and ascorbic acid (0.17 mmol, 30 mg) was mixed in DMF (1 mL) and the final solution was stirred at 70 °C for 24 h.The precipitate was separated employing an external magnet and washed repeatedly with distilled water (20 mL) and ethyl acetate (20 mL) and dried at 70 °C for 24 h.

Synthesis of Fe 3 O 4 @Phos-IL-AuCu
A suspension of Fe 3 O 4 @Phos-IL (300 mg) in distilled water (6 mL) was sonicated for 15 min, and then a solution of CuSO    www.nature.com/scientificreports/added slowly and the reaction stirred for 1 day at 25 °C.Finally, the resulting solid was separated and treated as it was described above.www.nature.com/scientificreports/by GC analysis and stopped when it was judged complete.The crude substance was treated with ethyl acetate (3 × 5 mL), and the organic residue was purified by chromatography (column or plate).

Catalytic reduction of nitro compounds
General procedure for the catalytic reduction of MO, MR, and RhB dyes 2.5 mL of the desired dye's solution (MO, MR, or RhB, 0.06 mM), the catalyst (1 mg) and 0.5 mL of an aqueous solution of NaBH 4 (0.5 M) were poured into a quartz cuvette, and the mixture was stirred at room temperature.UV-Visible spectroscopy was selected to monitor the progress of the reaction.
Typical procedures for the degradation of TC TC (0.005 mmol, 2.5 mg), catalyst (15 mg), and H 2 O (50 mL) were poured into an Erlenmeyer flask and stirred for 3 min, and then ammonium peroxydisulfate (1 mmol, 240 mg) was added to the mixture.Again, the course of the reaction was monitored every 10 min through UV-Visible spectroscopy.

One-pot synthesis of acetaminophen
In a 5 mL flask, 4-nitrophenol (0.4 mmol, 55.6 mg), NaBH 4 (1.6 mmol, 60.5 mg), catalyst (20 mg), and H 2 O (1.5 mL) were added and the reaction was stirred at 25 °C.After 2 h, acetic anhydride (1 mmol, 0.1 mL) was poured into the flask, and the reaction mixture was stirred for an additional 2 hours at 100 °C.The crude material was extracted with ethyl acetate (3×5 mL), purified by column/plate chromatography, and acetaminophen was obtained in 84% isolated yield.It was characterized only by 1 H NMR.

Conclusion
In conclusion, a new magnetic and recyclable bimetallic Fe 3 O 4 @Phos-IL-AuCu catalyst was synthesized.This catalyst showed high activity in reducing nitro compounds, reduction of organic dyes, and degrading tetracycline.It also promoted all three of these reactions at room temperature in water as a green solvent.The Fe 3 O 4 @ Phos-IL-AuCu catalyst showed the highest activity in comparison with its single metal species such as Fe 3 O 4 @ Phos-IL-Au, and Fe 3 O 4 @Phos-IL-Cu in all three reactions mentioned, which confirms the synergistic effects between gold and copper particles.In addition, the synthesized catalyst can be easily recycled due to its magnetic properties and reused up to 17 times without noticeable decrease in its activity.

Figure 8 .Figure 9 .Scheme 2 .
Figure 8. UV-Visible spectra for the reductive degradation of (a) MO, (b) MR, and (c) RhB in the presence of bimetallic Fe 3 O 4 @Phos-IL-AuCu catalyst and NaBH 4 and (d) diagrams of ln (At/A0) versus reaction time for the reduction of listed dyes.

Figure 12 .
Figure 12.XPS spectrum of the reused catalyst after 17th run in Fe, Si, C, N, Cl, P, Au, and Cu regions.

Figure 13 .
Figure 13.XRD pattern of the catalyst after 17th run.